1. Field of the Invention
The present invention relates to image output apparatuses used for printing high-quality images, particularly high-quality monochrome images, and image output methods.
2. Description of the Related Art
In general, a color inkjet printer as an example of a color output apparatus uses four colors including three primary colors, that is, cyan (C), magenta (M), and yellow (Y), and black (BK) to form an image.
Japanese Patent Laid-Open No. 2005-238835 discloses an example of a printing technique of black-and-white photos using a color inkjet printer. In this technique, an achromatic recording material is mainly used and one or more colors of chromatic recording materials are used as color-tone components whereby display of color tones is realized even for a monochrome image. In addition, even for an image in which a color tone is realized, use of the technique for forming the image prevents so-called color misregistration from being generated in which color saturation of an image in a calorimetric system and color saturation of an image recorded using recording materials are different from each other and prevents color transition from being generated in which transition of recording color partly occurs in uniform gradation or in uniform hue shift. Accordingly, high-quality printing is achieved when a black-and-white photo in which a color tone thereof is emphasized is printed.
Furthermore, according to Japanese Patent Laid-Open No. 2006-86708, in the second and subsequent steps of color separation processing, an available quantity of the chromatic color recording material is limited to one half or less of available quantity of achromatic recording material whereby occurrence of the color transition is reduced. Furthermore, since use of chromatic components is controlled with high accuracy, high-quality monochrome printing is achieved.
Human Eyes are sensitive to a gray color and similar colors thereof and more sensitive to color differences among colors of low color saturation than color differences among colors of high color saturation. Therefore, there is a strong demand for printing, with high-color accuracy, of an image including color components of low color saturation, for example, printing of a monochrome photo mainly including a gray color and similar hues thereof. Furthermore, in recent years, there has been a demand for a function of outputting a monochrome photo which can be output after tuning a color tone thereof so that a color tone desired by a user, such as a neutral tone, a warm tone, or a cool tone, can be attained. (This function is hereinafter called color-tone tuning.)
Furthermore, according to an image processing method disclosed in Japanese Patent Laid-Open No. 2006-86708, a high-quality monochrome image is output. However, although a significant effect is attained when this image processing method is applied to an image output apparatus used for outputting a monochrome image having a fixed color tone, only a small effect is attained when this image processing method is applied to an image output apparatus capable of performing color-tone tuning. According to Japanese Patent Laid-Open No. 2006-86708, in the image output apparatus used for outputting a monochrome image having a fixed color tone, a chromatic color component is added to a monochrome image having a fixed color tone. A maximum value of an available quantity of the chromatic color component is limited and a quantity of a color material increased when a signal value is increased by 1 is reduced whereby the color material is controlled with high accuracy. On the other hand, the image output apparatus capable of performing color-tone tuning may output a monochrome image as an image having higher color saturation by tuning a color tone thereof when compared with the image having a fixed color tone. In this case, a larger amount of chromatic color component is used when compared with the monochrome image having a fixed color tone. Accordingly, since a maximum amount of used chromatic color component becomes large, and therefore, an amount of the color material increased when the signal value is increased by 1 relating to the limitation of use amount is increased, the accuracy of control of the color material is deteriorated.